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On x86, x86-64 isn't significantly different just more complex. The registers have "R" instead of "E" which probably stood for Extended. The addressing modes are different with things like IP relative addressing. In fact with prefixes, 32 bit operand and/or address forms are also possible. Many reversing concepts can be learned on 32 bit code. But if you want to get familiarity and practical modern experience then might as well go with 64 bit. Even apps that swore they would never switch to 64 bit like Visual Studio have by now made the switch. Intel is even trying to deprecate the 32 bit features but they dropped their effort after forming a standardization group, possibly to resurface there. The legacy compatibility really wastes valuable chip real estate on x86 cpus. Personally I'd rather have AVX512 or powerful extensions than a CPU that has to boot into 16 bit real mode then switch to 32 bit virtual mode before finally switching to 64 bit mode.
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